This invention is generally directed to processes for the preparation of carrier particles, and more specifically, the present invention relates to a sequential addition process for the preparation of coated carrier particles. In one embodiment of the present invention the carrier particles are prepared by the mixing of a carrier core with a first polymeric carrier coating; thereafter mixing a second carrier coating; and heat treating the resulting components until the first and second polymer are fused to the core. Another embodiment of the present invention is directed to a simple economical process for the preparation of carrier particles which comprises the initial mixing of a carrier core with a first polymeric carrier coating to form a premixture; thereafter mixing a second polymeric carrier coating with the premixture wherein the second polymer is not in close proximity to the first polymer in the triboelectric series; and heat treating the resulting components in, for example, a rotary kiln until the first and second polymer are fused to the core. With the processes of the present invention there are enabled in several embodiments thereof insulating carrier particles with relatively constant conductivity parameters, and also wherein the triboelectric charge on the carrier can vary significantly depending on the coatings selected. Developer compositions comprised of the carrier particles prepared by the process of the present invention are useful in electrostatographic or electrophotographic imaging and printing systems, especially xerographic imaging processes. Additionally, developer compositions comprised of substantially insulating carrier particles prepared in accordance with the process of the present invention are useful in imaging methods wherein relatively constant conductivity parameters are desired. Furthermore, in the aforementioned imaging processes the triboelectric charge on the carrier particles can be preselected depending on the polymer composition applied to the carrier core.
The electrostatographic process, and particularly the xerographic process, is well known. This process involves the formation of an electrostatic latent image on a photoreceptor, followed by development, and subsequent transfer of the image to a suitable substrate. Numerous different types of xerographic imaging processes are known wherein, for example, insulative developer particles or conductive toner compositions are selected depending on the development systems used. Moreover, of importance with respect to the aforementioned developer compositions is the appropriate triboelectric charging values associated therewith, as it is these values that enable continued constant developed images of high quality and excellent resolution.
Additionally, carrier particles for use in the development of electrostatic latent images are described in many patents including, for example, U.S. Pat. No. 3,590,000. These carrier particles may comprise various cores, including steel, with a coating thereover of fluoropolymers, and terpolymers of styrene, methacrylate, and silane compounds. Efforts have focused on the attainment of coatings for carrier particles, for the purpose of improving development quality, and also to permit particles that can be recycled, and that do not adversely effect the imaging member in any substantial manner. Many of the present commercial coatings can deteriorate rapidly, especially when selected for a continuous xerographic process where the entire coating may separate from the carrier core in the form of chips or flakes; and fail upon impact, or abrasive contact with machine parts and other carrier particles. These flakes or chips, which cannot generally be reclaimed from the developer mixture, have an adverse effect on the triboelectric charging characteristics of the carrier particles thereby providing images with lower resolution in comparison to those compositions wherein the carrier coatings are retained on the surface of the core substrate. Further, another problem encountered with some prior art carrier coatings resides in fluctuating triboelectric charging characteristics, particularly with changes in relative humidity. The aforementioned modification in triboelectric charging characteristics provides developed images of lower quality, and with background deposits.
There are also illustrated in U.S. Pat. No. 4,233,387, the disclosure of which is totally incorporated herein by reference, coated carrier components for electrostatographic developer mixtures comprised of finely divided toner particles clinging to the surface of the carrier particles. Specifically, there are disclosed in this patent coated carrier particles obtained by mixing carrier core particles of an average diameter of from between about 30 microns to about 1,000 microns with from about 0.05 percent to about 3.0 percent by weight, based on the weight of the coated carrier particles, of thermoplastic resin particles. The resulting mixture is then dry blended until the thermoplastic resin particles adhere to the carrier core by mechanical impaction, and/or electrostatic attraction. Thereafter, the mixture is heated to a temperature of from about 320.degree. F. to about 650.degree. F. for a period of 20 minutes to about 120 minutes enabling the thermoplastic resin particles to melt and fuse on the carrier core. While the developer and carrier particles prepared in accordance with the process of this patent, the disclosure of which has been totally incorporated herein by reference, are suitable for their intended purposes, the conductivity values of the resulting particles are not constant in all instances, for example, when a change in carrier coating weight is accomplished to achieve a modification of the triboelectric charging characteristics; and further with regard to the '387 patent, in many situations carrier and developer mixtures with only specific triboelectric charging values can be generated when certain conductivity values or characteristics are contemplated. With the invention of the present application, the conductivity of the resulting carrier particles are substantially constant, and moreover the triboelectric carrier values can be selected to vary significantly, for example, from less than -15 microcoulombs per gram to greater than -70 microcoulombs per gram, depending on the polymer mixture selected for affecting the coating process.
Additionally, it is known that carrier particles with a first and second coating, which coatings are not in close proximity in the triboelectric series, can be prepared by initially mixing the coatings followed by further mixing with a carrier core, and subsequently heat treating the resulting components until the polymers fuse and adhere to the carrier core. More specifically, the aforementioned processes are disclosed in U.S. Pat. No. 4,935,326 and 4,937,166, the disclosures of each of these applications being totally incorporated herein by reference, the Japanese equivalent of U.S. Ser. No. 793,042 (now abandoned) was published on May 16, 1987 as Japanese Publication 106475/87, the disclosure of which is totally incorporated herein by reference. In U.S. Pat. No. 4,937,166 there are disclosed developer compositions comprised of toner particles, and carrier particles prepared by a powder coating process; and wherein the carrier particles consist of a core with a coating thereover comprised of a mixture of polymers. More specifically, the carrier particles selected can be prepared by mixing low density porous magnetic, or magnetically attractable metal core carrier particles with from, for example, between about 0.05 percent and about 3 percent by weight, based on the weight of the coated carrier particles, of a mixture of polymers until adherence thereof to the carrier core by mechanical impaction or electrostatic attraction; heating the mixture of carrier core particles and polymers to a temperature, for example, of between from about 200.degree. F. to about 550.degree. F. for a period of from about 10 minutes to about 90 minutes enabling the polymers to melt and fuse to the carrier core particles; cooling the coated carrier particles; and thereafter classifying the obtained carrier particles to a desired particle size. One disadvantage associated with the aforementioned processes resides in the need for one additional blender, which disadvantage is avoided with the processes of the present invention. Other advantages associated with the processes of the present invention include, for example, (1) avoiding the storage of premixes of the polymer selected; (2) the ratio amounts of each of the polymer selected can be instantly modified during the preparation thereof; (3) triboelectric charging properties and particularly the triboelectric charging values of the carriers can be modified and controlled depending on the mixing time of the components; (4) reduced cost in some instances in obtaining carrier particles as compared, for example, to the processes as disclosed in the patents mentioned herein; (5) any environmental adverse effects on the polymers is eliminated or minimized; and (6) dust generated with the processes of the prior art, such as illustrated by the processes in the patents, is eliminated or minimized during mixing of the first and second polymers.
In a patentability search report the following U.S. Pat. Nos. were recited: 4,233,387 directed to coating carrier particles by dry blending with resin particles including Kynar, that is polyvinylidene fluoride, and causing the polymer particles to adhere to the carrier by melt fusing; U.S. Pat. Nos. 4,297,427 and 4,725,521 which describe the concept of coating carrier particles with, for example, a mixture of Kynar and an acrylate, note the Abstract of the Disclosure of the '427 patent wherein carrier particles comprising a core have been an outer coating thereon comprising a polyblend of the first polymer possessing negative triboelectric charging characteristics with respect to the toner particles and a second polymer which possesses strong adhesive properties with respect to the core of the carrier particles; and also see the disclosure beginning at column 3, and continuing on to column 5, and note the examples of first polymer in column 4, beginning at line 26, and examples of second polymers beginning at column 4, line 36; and of background interest U.S. Pat. Nos. 3,916,065; 3,923,503; 4,304,830; 4,310,611 and 4,524,120.
With further reference to the prior art, carriers obtained by applying insulating resinous coatings to porous metallic carrier cores using solution coating techniques are undesirable from many viewpoints. For example, the coating material will usually reside in the pores of the carrier cores, rather than at the surfaces thereof; and therefore is not available for triboelectric charging when the coated carrier particles are mixed with finely divided toner particles. Attempts to resolve this problem by increasing the carrier coating weights, for example, to as much as 3 percent or greater to provide an effective triboelectric coating to the carrier particles necessarily involves handling excessive quantities of solvents, and further usually these processes result in low product yields. Also, solution coated carrier particles when combined and mixed with finely divided toner particles provide in some instances triboelectric charging values which are too low for many uses. The dual solution coating process of the present invention overcomes and/or minimizes these disadvantages, and further enables developer mixtures that are capable of generating high and useful triboelectric charging values with finely divided toner particles; and also wherein the carrier particles are of substantially constant conductivity. Additionally, there can be achieved with the process of the present invention, independent of one another, desirable triboelectric charging characteristics and conductivity values; that is, for example the triboelectric charging parameter is not dependent on the carrier coating weight as is believed to be the situation with the process of U.S. Pat. No. 4,233,387 wherein an increase in coating weight on the carrier particles may function to also permit an increase in the triboelectric charging characteristics. Specifically, therefore, with the carrier compositions and process of the present invention there can be formulated developers with selected triboelectric charging characteristics and/or conductivity values in a number of different combinations.
Thus, for example, there can be formulated in accordance with the invention of the present application developers with conductivities of from about 10.sup.-6 mho (cm).sup.-1 to 10.sup.-17 mho (cm).sup.-1 as determined in a magnetic brush conducting cell; and triboelectric charging values of from about a -8 to a -80 microcoulombs per gram on the carrier particles as determined by the known Faraday Cage technique. Thus, developers containing carrier particles prepared by the process of the present invention can be formulated with constant conductivity values with different triboelectric charging characteristics by, for example, maintaining the same coating weight on the carrier particles and changing the polymer coating ratios. Similarly, there can be formulated developer compositions wherein constant triboelectric charging values are achieved and the conductivities are altered by retaining the polymer ratio coating constant and modifying the coating weight for the carrier particles.
Other patents of interest include U.S. Pat. No. 3,939,086, which teaches steel carrier beads with polyethylene coatings, see column 6; U.S. Pat. No. 4,264,697, which discloses dry coating and fusing processes; U.S. Pat. Nos. 3,533,835; 3,658,500; 3,798,167; 3,918,968; 3,922,382; 4,238,558; 4,310,611; 4,397,935 and 4,434,220.